Prog Med Phys.  2021 Mar;32(1):18-24. 10.14316/pmp.2021.32.1.18.

Feasibility of Improving the Accuracy of Dose Calculation Using Hybrid Computed Tomography Images: A Phantom Study

• Jeon H
• Kim DW
• Joo JH
• Ki Y
• Kim W
• Park D
• Nam J
• Kim DH

• KMID: 2514822
• DOI: http://doi.org/10.14316/pmp.2021.32.1.18

Abstract

Purpose
Kilovoltage computed tomography (kV-CT) is essential for radiation treatment planning. However, kV-CT images are significantly distorted by artifacts when a metallic prosthesis is present in the patient's body. Thus, the accuracies of target delineation and treatment dose calculation are inevitably lowered. We evaluated the accuracy of the calculated doses using an image restoration method with hybrid CT, which was introduced in our previous study.
Methods
A cylindrical phantom containing four metals, namely, silver, copper, tin, and tungsten, was scanned using kV-CT and megavoltage CT to produce hybrid CT images. We created six verification plans for three head and neck patients on kV-CT and hybrid CT images of the phantom and calculated their doses. The actual doses were measured with film patches during beam delivery using tomotherapy. We used the gamma evaluation method to compare dose distribution between kV-CT and hybrid CT with three gamma criteria, namely, 3%/3 mm, 2%/2 mm, and 1%/1 mm.
Results
The gamma pass rates decreased as the gamma criteria were strengthened, and the pass rate of hybrid CT was higher than that of kV-CT in all cases. When the 1%/1 mm criterion was used, the difference in gamma pass rates between them was up to 13%p.
Conclusions
According to our findings, we expect that the use of hybrid CT can be a suitable approach to avoid the effect of severe metal artifacts on the accuracy of dose calculation and contouring.

Keyword

Metal artifact; Hybrid computed tomography; Radiation treatment planning; Dose calculation

Figure

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  Fig. 1 Experimental setup of (a) the cylindrical phantom containing (b) four metal inserts. (c) Transverse view of the phantom. Ag, silver; Cu, Copper; Sn, tin; W, tungsten.

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  Fig. 2 Density distributions of the phantom (a) scanned using kilovoltage computed tomography (kV-CT) and (b) generated using hybrid CT, expressed in g/cm3. (c, d) They are also plotted on three-dimensional axes. Ag, silver; Cu, Copper; Sn, tin; W, tungsten.

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  Fig. 3 Density-intensity curves in the reconstructed images of kilovoltage computed tomography (kV-CT) and hybrid CT.

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  Fig. 4 Calculated dose distributions on the phantom obtained using (a) kilovoltage computed tomography (kV-CT) and (b) hybrid CT in the coronal view in patient 1.

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  Fig. 5 Dose distributions calculated using (a) kilovoltage computed tomography (kV-CT) and (b) hybrid CT, and (c) measured using film in the coronal view in patient 1. The size of the film is 200×30 mm2.

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  Fig. 6 Comparison of gamma pass rates at different gamma criteria in the three patients. kV-CT, kilovoltage computed tomography; HCT, hybrid CT.

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  Fig. 7 Gamma evaluation results of (a–c) kilovoltage computed tomography (kV-CT) and (d–f) hybrid CT with the three gamma criteria in patient 1.

Reference

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